1. Field of the Invention
This invention relates to methods and apparatus for filtering signals utilizing a vibrating micromechanical resonator.
2. Background Art
The need for passive off-chip components has long been a key barrier against communication transceiver miniaturization. In particular, the majority of the high-Q bandpass filters commonly used in the RF and IF stages of heterodyning transceivers are realized using off-chip, mechanically-resonant components, such as crystal and ceramic filters and SAW devices, as illustrated in FIG. 1. Due to higher quality factor Q, such technologies greatly outperform comparable filters implemented using transistor technologies, in insertion loss, percent bandwidth, and achievable rejection. High Q is further required to implement local oscillators or synchronizing clocks in transceivers, both of which must satisfy strict phase noise specifications. Again, as illustrated in FIG. 1, off-chip elements (e.g., quartz crystals) are utilized for this purpose.
Being off-chip components, the above mechanical devices must interface with integrated electronics at the board level, and this constitutes an important bottleneck against the miniaturization of super-heterodyne transceivers. For this reason, recent attempts to achieve single-chip transceivers for paging and cellular communications have utilized alternative architectures that attempt to eliminate the need for off-chip high-Q components via higher levels of transistor integration. Unfortunately, without adequate front-end selectivity, such approaches have suffered somewhat in overall performance, to the point where they so far are usable only in less demanding applications.
Given this, and recognizing that future communication needs will most likely require higher levels of performance, single-chip transceiver solutions that retain high-Q components and that preserve super-heterodyne-like architectures are desirable.
Recent demonstrations of vibrating beam micromechanical (xe2x80x9cxcexcmechanicalxe2x80x9d) resonator devices with frequencies in the VHF range and Q""s in the tens of thousands have sparked a resurgence of research interest in communication architectures using high-Q passive devices as disclosed in the above-noted patent application entitled xe2x80x9cDevice Including A Micromechanical Resonator Having An Operating Frequency and Method of Extending Same.xe2x80x9d Much of the interest in these devices derives from their use of IC-compatible microelectromechanical systems (MEMS) fabrication technologies to greatly facilitate the on-chip integration of ultra-high-Q passive tanks together with active transistor electronics, allowing substantial size reduction.
FIG. 2 illustrates a comparison of MEMS and SAW technologies wherein MEMS offers the same or better high-Q frequency selectivity with orders of magnitude smaller size. Indeed, reductions in size and board-level packaging complexity, as well as the desire for the high performance attainable by super-heterodyne architectures, are principal drivers for this technology.
Although size reduction is certainly an advantage of this technology (commonly dubbed xe2x80x9cRF MEMSxe2x80x9d), it merely touches upon a much greater potential to influence general methods for signal processing. In particular, since they can now be integrated (perhaps on a massive scale) using MEMS technology, vibrating xcexcmechanical resonators (or xcexcmechanical links) can now be thought of as tiny circuit elements, much like resistors or transistors, in a new mechanical circuit technology. Like a single transistor, a single mechanical link does not possess adequate processing power for most applications. However, again like transistors, when combined into larger (potentially, VLSI) circuits, the true power of xcexcmechanical links can be unleashed, and signal processing functions with attributes previously inaccessible to transistor circuits may become feasible.
The Need for High Q in Oscillators
For any communications application, the stability of the oscillator signals used for frequency translation, synchronization, or sampling, is of utmost importance. Oscillator frequencies must be stable against variations in temperature against aging, and against any phenomena, such as noise or microphonics, that cause instantaneous fluctuations in phase and frequency. The single most important parameter that dictates oscillator stability is the Q of the frequency-setting tank (or of the effective tank for the case of ring oscillators). For a given application, and assuming a finite power budget, adequate long- and short-term stability of the oscillation frequency is insured only when the tank Q exceeds a certain threshold value.
Given the need for low power in portable units, and given that the synthesizer (containing the reference and VCO oscillators) is often a dominant contributor to total transceiver power consumption, modern transceivers could benefit greatly from technologies that yield high-Q tank components.
The Need for High Q in Filters
Tank Q also greatly influences the ability to implement extremely selective IF and RF filters with small percent bandwidth, small shape factor, and low insertion loss. As tank Q decreases, insertion loss increases very quickly, too much even for IF filters, and quite unacceptable for RF filters. As with oscillators, high-Q tanks are required for RF and IF filters alike, although more so for the latter, since channel selection is done predominantly at the IF in super-heterodyne receivers. In general, the more selective the filter, the higher the resonator Q required to achieve a given level of insertion loss.
Micromechanical Circuits
Although mechanical circuits, such as quartz crystal resonators and SAW filters, provide essential functions in the majority of transceiver designs, their numbers are generally suppressed due to their large size and finite cost. Unfortunately, when minimizing the use of high-Q components, designers often trade power for selectivity (i.e., Q), and hence, sacrifice transceiver performance. As a simple illustration, if the high-Q IF filter in the receive path of a communication subsystem is removed, the dynamic range requirement on the subsequent IF amplifier, IQ mixer, and A/D converter circuits, increases dramatically, forcing a corresponding increase in power consumption. Similar trade-offs exist at RF, where the larger the number or greater the complexity of high-Q components used, the smaller the power consumption in surrounding transistor circuits.
The Micromechanical Beam Element
To date, the majority of xcexcmechanical circuits most useful for communication applications in the VHF range have been realized using xcexcmechanical flexural-mode beam elements, such as shown in FIG. 2 with clamped-clamped boundary conditions. Although several micromachining technologies are available to realize such an element in a variety of different materials, surface micromachining has been the preferred method for xcexcmechanical communication circuits, mainly due to its flexibility in providing a variety of beam end conditions and electrode locations, and its ability to realize very complex geometries with multiple levels of suspension.
U.S. Pat. No. 6,049,702 to Tham et al. discloses an integrated passive transceiver section wherein microelectromechanical (MEM) device fabrication techniques are used to provide low loss, high performance switches. Utilizing the MEM devices also makes possible the fabrication and use of several circuits comprising passive components, thereby enhancing the performance characteristics of the transceiver.
U.S. Pat. No. 5,872,489 to Chang et al. discloses an integrated tunable inductance network and method. The network utilizes a plurality of MEM switches which selectively interconnect inductance devices thereby providing a selective inductance for a particular circuit.
U.S. Pat. No. 5,963,857 to Greywall discloses an article comprising a micromachined filter. In use, the micromachined filters are assembled as part of a radio to miniaturize the size of the radio.
U.S. Pat. Nos. 5,976,994 and 6,169,321 to Nguyen et al. disclose a batch-compatible, post-fabrication annealing method and system to trim the resonance frequency and enhance the quality factor of micromechanical structures.
U.S. Pat. Nos. 5,455,547; 5,589,082 and 5,537,083 to Lin et al. disclose microelectromechanical signal processors. The signal processors include many individual microelectromechanical resonators which enable the processor to function as a multi-channel signal processor or a spectrum analyzer.
U.S. Pat. No. 5,640,133 to MacDonald et al. discloses a capacitance-based, tunable, micromechanical resonator. The resonators may be selectively tuned and used in mechanical oscillators, accelerometers, electromechanical filters and other electronic devices.
U.S. Pat. No. 5,578,976 to Yao, U.S. Pat. No. 5,619,061 to Goldsmith et al. and U.S. Pat. No. 6,016,092 to Qiu et al. disclose various micromechanical and microelectromechanical switches used in communication apparatus.
U.S. Pat. No. 5,839,062 to Nguyen et al. disclose a MEMS-based receiver including parallel banks of microelectromechanical filters.
U.S. Pat. Nos. 5,491,604 and 5,955,932 to Nguyen et al. disclose Q-controlled microresonators and tunable filters using the resonators.
U.S. Pat. No. 5,783,973 to Weinberg et al. discloses a micromechanical, thermally insensitive silicon resonator and oscillator.
The following articles are of general interest: Nguyen et al., xe2x80x9cDesign and Performance of CMOS Micromechanical Resonator Oscillatorsxe2x80x9d, 1994 IEEE International Frequency Control Symposium, pp. 127-134; Wang et al, xe2x80x9cQ-Enhancement of Microelectromechanical Filters Via Low-Velocity Spring Couplingxe2x80x9d, 1997 IEEE Ultrasonics Symposium, pp. 323-327; Bannon, III et al., xe2x80x9cHigh Frequency Microelectromechanical IF Filtersxe2x80x9d, 1996 IEEE Electron Devices Meeting, San Francisco, Calif., Dec. 8-11, 1996, pp. 773-776; and Clark et al., xe2x80x9cParallel-Resonator HF Micromechanical Bandpass Filtersxe2x80x9d 1997 International Conference On Solid-State Sensors And Actuatorsxe2x80x9d, pp. 1161-1164.
U.S. Pat. No. 5,640,133 to MacDonald et al. discloses a capacitance-based tunable micromechanical resonator. The resonator includes a movable beam which holds a plurality of electrodes. The resonator also includes a plurality of stationary electrodes. In operation, an adjustable bias voltage, applied to the beam electrodes and the stationary electrodes, is used to adjust the resonant frequency of the resonator.
U.S. Pat. No. 5,550,516 to Burns et al. discloses an integrated resonant microbeam sensor and transistor oscillator. The sensor and oscillator, capable of providing high-Q values, utilizes various circuitry, electrode placement, and various configurations of microbeam geometry to vary the operating resonant frequency.
U.S. Pat. No. 5,399,232 to Albrecht et al. discloses a microfabricated cantilever stylus with an integrated pyramidal tip. The pyramidal tip, integrally formed on the cantilever arm, limits the movement of the arm in the direction of the tip.
U.S. Pat. No. 4,262,269 to Griffin et al. discloses a Q-enhanced resonator which utilizes resonator positioning to provide a desired performance. Resonators are separated by one-quarter-wavelength distances to obtain desired loss characteristics.
U.S. Pat. No. 4,721,925 to Farace et al. discloses a micromechanical electronic oscillator etched from a silicon wafer. The patent discusses the configuration and the circuitry which enables the oscillator to perform according to desired characteristics.
The following U.S. patents are generally related to this invention: U.S. Pat. Nos. 4,081,769; 4,596,969; 4,660,004; 4,862,122; 5,065,119; 5,191,304, 5,446,729; 5,428,325, 5,025,346; 5,090,254; 5,455,547; 5,491,604; 5,537,083; and 5,589,082.
An object of the present invention is to provide a method and apparatus for filtering signals utilizing a vibrating micromechanical resonator wherein the resonator is isolated from a support structure for the resonator during resonator vibration.
In carrying out the above object and other objects of the present invention, a method is provided for filtering signals to obtain a desired passband of frequencies. The method includes providing a micromechanical filter apparatus including a micromechanical resonator having a fundamental resonant mode formed on a substrate and a support structure anchored to the substrate to support the resonator above the substrate. The method also includes vibrating the resonator so that the apparatus passes a desired frequency range of signals while substantially attenuating signals outside the desired frequency range. The support structure is attached to the resonator so that the resonator is isolated from the support structure during resonator vibration.
The step of vibrating may include forcing different portions of the resonator to move in opposite directions at the same time so that the resonator vibrates in a resonant mode, m, higher than the fundamental resonant mode wherein the resonator has m+1 nodal points.
The micromechanical filter apparatus may include a plurality of input electrodes spaced along the resonator to allow electrostatic excitation of the resonator. The step of forcing may include the steps of applying an in-phase signal to one of the input electrodes to deflect a first portion of the resonator in a first direction and applying an out-of-phase signal to another input electrode to deflect a second portion of the resonator in a second direction opposite the first direction to force the resonator into a correct mode shape.
The micromechanical filter apparatus may include an input electrode formed on the substrate to allow electrostatic excitation of the resonator. The step of forcing may include the step of applying a signal to the input electrode. The resonator and the input electrode may define a capacitive transducer gap therebetween. The resonator may further include m+1 spacers having a height and which extend between the resonator and the substrate at the m+1 nodal points. The m+1 spacers force the resonator into a correct mode shape during the application of the signal to the input electrode.
Further, in carrying out the above object and other objects of the present invention a micromechanical filter apparatus for filtering signals to obtain a desired passband of frequencies is provided. The apparatus includes a substrate, a plurality of intercoupled micromechanical elements including a resonator and a support structure anchored to the substrate to support the elements above the substrate. The support structure and the resonator are both dimensioned so that the resonator is isolated from the support structure during resonator vibration. Energy losses to the substrate are substantially eliminated. The apparatus is a high-Q apparatus.
The support structure may be attached to the resonator at at least one nodal point of the resonator.
The signals may be RF signals.
The apparatus may be an RF filter apparatus.
The apparatus may be a bandpass filter apparatus.
The support structure may include at least one beam attached to a nodal point of the resonator.
The apparatus may further include at least one input electrode formed on the substrate to allow electrostatic excitation of the resonator wherein the resonator and the at least one input electrode define a capacitive transducer gap therebetween.
The apparatus may further include at least one spacer having a height. Each spacer extends between the resonator and the substrate at a nodal point of the resonator. The size of the gap is based on the height of the at least one spacer during pull down of the resonator.
The apparatus may be a silicon-based filter apparatus.
The apparatus may be a diamond-based filter apparatus.
The apparatus may further include at least one output electrode formed on the substrate to sense output of the apparatus.
The support structure may include a plurality of beams and the resonator may include a plurality of nodal points. Each of the beams is attached to the resonator at one of the nodal points of the resonator so that the resonator sees substantially no resistance to transverse or torsional motion from the support structure.
A pair of balanced input electrodes may be formed on the substrate to allow electrostatic excitation of the resonator.
A pair of balanced output electrodes may be formed on the substrate to sense output of the apparatus.
The plurality of intercoupled micromechanical elements may include a pair of intercoupled end resonators.
The support structure may support the end resonators above the substrate.
The plurality of intercoupled micromechanical elements may include an inner resonator intercoupled to the end resonators.
The support structure may support the end and inner resonators above the substrate.
The plurality of intercoupled micromechanical elements may further include a plurality of coupling links for coupling the inner resonator to the end resonators.
The coupling links may be operable in multiple modes.
The coupling links may be higher mode coupling beams.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.